People are generally aware of the most common causes of hallucinations, like schizophrenia and a really high fever. But lesser known are the more rare causes; here are six you probably haven't heard about.

Although naps are a time-honored tradition among little children and the elderly, they have only become trendy since about the time Google added sleeping pods to its employee perks. As anyone of a skeptical nature might ask, Is taking a mid-day nap truly effective at boosting employee productivity? That has yet to be proven, but a recent study examining the effects on school children found naps to be beneficial for memory retention. Compared to children who continued their usual lessons after hearing a 15-minute lecture, students who napped retained more of what they’d learned up to five days later. “The results suggest that sleep can be used to enhance the duration of memory contents learned in school,” wrote the authors in their study.

Each of us has a natural chronotype — a time of day when our physical functioning, hormonal levels, and cognitive faculties are most active. It is this organic internal clock that leads us to be either night owls or morning larks. When those who are night owls must routinely wake up early, say, for a particular job, they experience so-called social jet lag, in that their natural rhythms are not in sync with their schedule. At first glance, social jet lag may appear to be a trivial variation on the sleep disturbances troubling people who travel from one time zone to another. Yet research has shown how social jet lag may increase your risk for obesity while also increasing your risk for depression. Social jet lag is said to be responsible for subtle difficulties with learning and memory, leading more than one scientist to suggest daily naps may be exactly what the doctor ordered for those out of sync with their lives, including busy Google employees.

“Social jetlag plays a key role in the health and functioning of adolescents,” wrote a team of Brazilian researchers. For this reason, they investigated whether post-learning daytime naps might benefit the formation of declarative memory — recollections we generally refer to as facts and knowledge — among adolescents. A total of 371 volunteer students between the ages of 10 and 15 participated in the study, which involved seven separate schools in the Brazilian city of Natal. All of the volunteers were healthy, with none using medication possibly affecting the sleep/wake cycle. Students were pre-tested on lecture-related contents before the lecture, were invited to nap for up to two hours, and after one, two, or five days received surprise tests with similar content but different wording and question order. What did the researchers discover?

Compared with their pre-test scores, all of the students, whether in the nap or non-nap group, showed a gain of about 10 percent in test scores on the subject material one day after the lecture. Yet, after two- and five-day intervals, this gain was sustained only by the nap group. For the students who did not nap, the new knowledge was entirely gone after just five days. “Delaying the time of school onset for adolescents and allowing them to nap between classes are relatively easy implementations with potential to reduce classroom sleepiness in a cost-effective manner,” concluded the authors.